1. Field of the Invention
The present invention relates to a light scanning optical system and image forming apparatus using the same, and a light scanning optical system and light scanning optical apparatus used for a laser beam printer or digital copying machine and, more particularly, to a multi-beam light scanning optical system which uses a plurality of light sources as a light source to achieve high-speed operation and high recording density, and an image forming apparatus using the multi-beam light scanning optical system.
2. Related Background Art
FIG. 9 shows the main scanning section of a conventional multi-beam light scanning optical system using a plurality of light sources. A plurality of light sources 21 are formed from a semiconductor laser having a plurality of light emitting points. Each of light beams emitted from the plurality of light sources is converted into a substantially parallel beam or convergent beam by a collimator lens 22. Each light beam is shaped in its sectional shape through an aperture stop 23 and converged only in the sub scanning direction by a cylindrical lens 24 so that an image like a focal line long in the main scanning direction is formed near a deflecting/reflecting surface 25a of a polygon mirror 25 serving as an optical deflector. Each light beam reflected/deflected and scanned by the polygon mirror 25 that is rotating in a direction indicated by an arrow A in FIG. 9 at a predetermined speed is focused, through an f-θ lens 26, into a spot on a surface 27 to be scanned (scanning surface 27), comprising a photosensitive drum or the like, and scanned in a direction of an arrow B in FIG. 9 at a predetermined speed. A BD optical system 28 detects a write start position. The BD optical system 28 comprises a BD slit 28a, BD lens 28b, and BD sensor (synchronous position detection element) 28c. 
In such a multi-beam scanning optical system, if the plurality of light sources are laid out vertically in the sub scanning direction, as shown in FIG. 10, the sub-scanning line interval on the scanning surface becomes much more than the recording density. To avoid this, normally, a plurality of light sources are obliquely laid out, as shown in FIG. 11, and a tilt angle δ is adjusted whereby the sub-scanning line interval on the scanning surface is accurately adjusted so as to match the recording density.
In the conventional light scanning optical system having the above arrangement, the plurality of light sources are obliquely laid out. For this reason, since light beams emitted from the plurality of light sources reach the reflecting surface of the polygon mirror at positions apart in the main scanning direction, and are reflected with different reflection angles by the polygon mirror, so that spots are formed on the scanning surface at positions apart in the main scanning direction, as shown in FIG. 12 (light beam A and light beam B).
Hence, in such a multi-beam light scanning optical system, image data are sent with a delay of a predetermined time δT such that the image forming positions of light beams from the light sources match a position where a light beam from a certain reference light source forms its image on the scanning target surface.
With the delay time δT, the polygon surface is set to be a surface 25′ with an angle corresponding to the delay time δT. At this time, the light beam is reflected in a direction B′, i.e., in the same direction as that of the light beam A, so that the spot forming positions of the two light beams match.
Assume that a focusing error in the main scanning direction occurs due to some reason (e.g., a positional error between the scanning surface and the optical unit that holds the optical system, an assembly error in assembling optical components in the optical unit, or the like). In this case, when the scanning surface 27 shifts to a position 27′, as is apparent from FIG. 12, the image forming position of each light beam shifts in the main scanning direction by δY.
Conventionally, when the image forming position of each of the light beams from the plurality of light sources shifts, as described above, the printing accuracy decreases, and the image quality degrades.
The focus shift/error in the main scanning direction occurs due to various factors, and they cannot be completely eliminated. Even a process of adjusting them requires cost. Recently, an optical system using a plastic material is often used as an f-θ lens from the viewpoint of cost. A plastic lens is manufactured by injection molding, and its surface accuracy is lower than an accuracy obtained by polishing an optical glass member. Especially, a plastic lens readily produces a convex error with respect to a design value at a portion of the lens and a concave error at another portion. A focus shift due to such a surface accuracy error cannot be corrected across the scanning target surface.
It is therefore very difficult to correct a degradation in image quality due to the image forming position shift between the light beams from the plurality of light sources.
In the above description, the number of light emitting points is 2 for simplicity. As can easily be understood, when the number of light emitting points increases to 3, 4, 5, 6, . . . , the value δY generated between light sources at two ends proportionally increases. That is, in the conventional multi-beam light scanning optical system, even when the number of light emitting points is increased to attain high-speed operation, the printing accuracy decreases, and the image quality degrades because the above-described image forming position shift between the light beams from the plurality of light sources increases, resulting in difficulty in achieving high-speed operation.